1. Field of the Invention
The present invention relates to a system for thermal and/or acoustic insulation of a tube and, more particularly of a tube intended to allow the outflow of fluids, for example hydrocarbons originating from any source, for example an oil deposit.
2. Description of Related Art
Acoustic insulation is of interest above all for tubes which are close to living environments: overhead and submarine pipelines or pipeline networks buried in the ground. By way of example, in the case of high flow rates with a significant quantity of gas, the tubes and their singularities, such as bends, tees, valves, etc., generate levels of sound which are detrimental to man and the environment. Although currently not very practical, acoustic insulation contributes to human comfort, occupational safety and environmental protection.
When an oil deposit is brought into production, hydrocarbons flow through a tube, referred to as the production string, from the bottom of the well to the surface. At the bottom of the well, the pressure and the temperature are relatively high, for example 100.degree. C. and 300 bar. When the hydrocarbons rise to the surface, this pressure and this temperature decrease, with the result that the output temperature of the well is, for example, of the order of 30.degree. C.
The effect of this decrease in the temperature of the hydrocarbons in the production string is to increase the viscosity and the weight of these hydrocarbons, which may lead to their flow being slowed down. Furthermore, the temperature decrease may sometimes cause the deposition of hydrates or paraffins on the wall of the string. If it accumulates in the tube, this deposit may cause serious operating problems, such as slowing down the hydrocarbons or completely obstructing the tube. In general, an operator desiring to avoid these risks has to treat this deposition phenomenon, either preventively by injecting a chemical product which inhibits the deposition, or remedially by wiping or scraping the tube using special equipment, or else by heating it using a means which may be available. In all cases, these operations lead to considerable expenditure. This type of problem also occurs in the pipelines which join a well head to a remote processing centre.
The requirement for the outflow to remain cold may also arise when, for example, water is injected into the deposit from the surface. It is sometimes beneficial for the water not to be heated by the surrounding formations along this path, for example in order to maintain its weight or to cause certain thermal phenomena which promote injectability into the deposit (weakening the rock as a result of the low temperature with a view to thermally fracturing it). In this case, thermally insulating the tube makes it possible to reduce the time taken for the thermal phenomenon to occur and to keep conditions more constant during shut-downs, or to protect, around the tube, all the equipment which may be sensitive to thermal variations (contraction of the jacket, fracture of the cementing, etc.).
Some wells are located in regions where the ground temperature is very low. It has already been proposed to equip the production string with an electrical heating system in order to keep the temperature of the outflows constant. However, production, implementation and power consumption for this type of installation are very expensive, which greatly limits its usage.
Fitting thermal insulation around a tube or a production string, optionally coupled to an electrical or other heating system, makes it possible to keep the temperature of the outflows at a high value during their transfer, thus reducing the deposits on the wall of the column and other problems associated with temperature. It is known to use double-jacketed tubes and to use pumping to introduce a liquid loaded with solid particles, or a liquid gel, between the two walls from one of the ends of the tube. In the case of a well, the two walls in question are inherently present, one formed by the tubing and the other by the jacket.
This formulation, intended to stop natural convection within the liquid occupying the space separating the two walls, is relatively inexpensive to use because it generally avoids an elaborate operation of withdrawing the piping. However, its thermal insulation performance is mediocre because of the high residual conductivity of the medium, which is of the order of 0.6 W/m.degree.C. when it is based on water and of the order of 13 W/m.degree.C. when it is based on hydrocarbons (oil, gas oil, etc.). Furthermore, long-term stability of these formulations at a temperature of above 85.degree. C. is difficult to obtain (sedimentation of the particles in the case of loaded liquids, loss of viscosity in the case of liquid gels).
It is also known to use a tube or a string with a double wall, constructed in sections in the workshop and in which, between the two walls, a high vacuum is made or a low-pressure rare gas (argon, xenon, or the like) is introduced. This type of tube affords a high degree of thermal insulation, but its complicated workshop construction is very expensive and its high external diameter considerably limits its usage. Furthermore, fitting it to a system which is already being used requires the existing tubes to be replaced, which considerably increases the cost of the insulation.